Like a mass moving in a straight line (such as a dropped bearing ball or ping-pong ball), a stationary spinning object (such as a spinning bicycle wheel on a truing stand) also has inertia. The object traveling in a straight line has translational inertia, while the stationary spinning object has rotational inertia. That is the analogy.

An object's rotational inertia depends on both the total mass of the object and the distribution of that mass around the axis of rotation. Mass farther away from the axis of rotation contributes far more than mass close to the axis of rotation.

Example: take the case of two wheels, identical except for one having a heavy rim and one having a lighter one. Once spinning at the same rate, all else being equal the wheel with the heavy rim will spin far longer before it comes to a stop - because it stored more energy (as rotational kinetic energy) and had far more rotational inertia than the wheel with the lighter rim. (The "flip side": the wheel with the heavier rim will require more energy input to spin up to the same rate of rotation as the lighter rimmed wheel.)

Calculating either inertia or stored kinetic energy is possible for both translation and rotation. However, calculation is significantly easier for translational inertia (m x v) or translational kinetic energy (1/2 x m x v^2) than either rotational inertia or rotational kinetic energy. In contrast, both rotational cases are dependent on size, shape, and distribution of mass around the chosen axis of rotation.